Applications to Industrial Quality Inspection - 3D Computer Vision: Efficient Methods and Applications - page 262

Fig. 6.19 ( a ) High dynamic range image of the flange, displayed at logarithmic scale. Three-
dimensional reconstruction is performed for the ring-shaped surface part. The depths of the in-
dicated dents were measured on the reconstructed surface profile and compared to ground truth
data. ( b ) Triangulated stereo reconstruction result. ( c ) SfPR, no stereo information. ( d ) Shape from
shading, no stereo information. Albedo estimated based on specular reflections according to ( 5.23 )
cording to ( 5.23 ), relying on specular reflections. The three-dimensional reconstruc-
tion is performed for the ring-shaped part only, as the neighbouring parts are situ-
ated in the shadow and only visible due to secondary reflections (Fig. 6.19 a is a high
dynamic range image displayed at logarithmic scale). What is more, the surface nor-
mals of the neighbouring parts are nearly orthogonal to the viewing direction. Our
goniometer setup for measuring the intensity and polarisation reflectance functions
does not cope with such an extreme viewing geometry, such that in this range the
reflectance function values are unknown. Furthermore, photometric surface recon-
struction techniques are most favourably applied when the view on the surface is
largely perpendicular (McEwen, 1991 ). Although the small-scale deformations of
the surface are clearly apparent in the SfPR result (cf. Fig. 6.19 c) and to a lesser
extent also in the shape from shading result (cf. Fig. 6.19 d), large-scale deviations
from the essentially flat true surface shape are apparent.
6.3.3.2 Results Obtained with the SfPRD Technique
The convergent stereo setup employed by d'Angelo and Wöhler ( 2006 , 2008 )was
calibrated with the automatic camera calibration system described by Krüger et al.
( 2004 ). After acquisition of the images, they were rectified to standard epipolar
geometry. Effectively, this results in typical disparity values of around 4000 pixels
at the object distance in the rectified stereo image pairs. Experiments with synthetic